Modern technology has created a demand for reliable communications systems requiring relatively low power usage but having the ability to span great distances. This has been accomplished in many communication systems by utilizing relatively narrow bandwidths and antenna systems which are highly directional. This permits high signal density to be achieved but creates other difficulties, in that transmitting and receiving antennas must be precisely aligned for reliable communication to occur. Thus, the system must detect the presence of the signal and initially align the antennas.
In many instances, the signal transmitted is of the direct sequence spread spectrum type, which has a known ##EQU2## characteristic, with signal energy distributed over a wider frequency band and having a maximum amplitude at the central frequency and dimenishing side peaks separated by null points. This is merely one example of the type of system in which a known instantaneous spectral distribution exists, and in which the presence of the signal must be affirmatively identified. A particular problem encountered in the modern state of the art is that the noise energy within the receiver bandwidth may be substantially greater than the signal energy. It may, for example, be desired to detect a signal which is only 20% of the amplitude of the noise, so that the IF signal-to-noise ratio is negative, and as low as -7 db. This condition can readily arise when receiver and transmitter stations are in constant movement relative to each other, at a substantial spatial separation, and with their respective antennas scanning through a conical uncertainty region attempting to acquire each other. It is usually necessary to effect acquisition when the narrow beam antennas are not in direct alignment and when the signal to noise ratio is negative, expressed in decibels. Because the system is constrained to use the predetermined bandwidth of the communication signal, the problem of identifying the signal presence in the attendant noise becomes even more severe.
Numerous prior art systems have been developed to determine the presence of a signal in a highly noisy environment. Many of the techniques developed have been highly successful but none can cope with the handicap placed on the system when the signal is a spread spectrum signal. For instance, in Freedman U.S. Pat. No. 3,605,029 on "Signal Detection Apparatus" issued Sept. 14, 1971 the concept is disclosed of a receiver for use in a system in which randomly varying target indicating signals from a transducer are detected in the presence of noisy signals which vary in amplitude between wide limits. The approach of Freedman is to apply the signal through parallel channels each having a band pass filter of a different frequency and demodulating the output of each filter to produce signals which can be compared to provide the difference between the root mean square value of the noise signal and the target indicating signal. This system functions satisfactoily when a reasonable signal-to-noise ratio exists but when the received signal has the spread spectrum characteristic and the signal-to-noise ratio becomes negative, the system cannot provide an indication of the presence of a signal due to variations in circuitry components and the resultant variations in detector characteristics.
J. Kubanoff, U.S. Pat. No. 3,605,012 on "Noise Cancellation Filter System" issued Sept. 14, 1971 is similar to the Freedman system in that it splits the signal into two channels and each channel incorporates a filter responsive to a different frequency. In Kubanoff, the outputs of the filters are integrated and then summed as in Freedman to provide an output indicative of the presence of a signal as indicated by the difference between the signal-to-noise amplitudes. This system suffers from the same deficiencies as Freedman in the presence of a spread spectrum signal because the very low signal-to-noise ratios result in the signal being masked by the variations in channel response.
Other techniques utilized in attempts to isolate a signal in the presence of noise which have proved satisfactory in various environments but cannot cope with spread spectrum problems of very low signal-to-noise ratio include Webb, U.S. Pat. No. 3,350,643, on "Signal-To-Noise Ratio Estimating by Taking Ratio of Mean and Standard Deviation of Integrated Signal Samples" issued Oct. 31, 1967 wherein a carrier and subcarrier are demodulated to provide outputs that are integrated, converted from analog-to-digital signals and applied to a computer means which produces a ratio. This system has functioned satisfactorily for space vehicle communications but fails to provide an adequate response when spread spectrum problems occur reducing the signal-to-noise ratio below the sensitivity of the analog-to-digital converters and computer means incorporated by the system.
L. Deerkoski, U.S. Pat. No. 3,737,781 on "Signal-To-Noise Ratio Determination Circuit" issued June 5, 1973 is a still further example of prior techniques utilized to identify the presence of a signal by producing an output which is a function of the difference between the noise signal and the signal plus noise. However, this system cannot cope with the spread spectrum problem due to the relatively low signal-to-noise ratios encountered.
H. Brendzel et al, U.S. Pat. No. 3,855,423 on "Noise Spectrum Equalizer" issued Dec. 17, 1974; Campbell, U.S. Pat. No. 3,821,482 on "Noise Spectrum Equalizer Utilizing Spectrum Inversion" issued June 28, 1974; and O'Connor, U.S. Pat. No. 3,611,145 issued Oct. 5, 1971 are examples of techniques utilized to overcome noise problems by various equalization techniques which work satisfactorily in most environments but fail to provide the type of response required to accurately indicate the presence of a signal when the signal-to-noise ratio is in the range of only a few percent as is experienced in the spread spectrum signal environment.